Battery chargers for providing a constant current charge to a battery have been known for many years. In such charging systems, a constant current is initially provided to the battery at a relatively high rate until the battery voltage reaches a predetermined threshold. When this threshold or cutoff voltage level is reached, the high charge rate is discontinued and the rate becomes a continuous trickle charge of relatively low charge rate. High charging rate automatically begins again if and when the battery voltage drops to a certain value. It is also known to provide such circuits with a controllable hysteresis loop to preclude constant, rapid switching between low and high charging rate about the threshold battery voltage.
One example of constant current battery charging circuits of this type is shown in U.S. Pat. No. 3,919,618 to Coleman et al. The charging circuit of Coleman et al. provides a voltage reference element diode for temperature compensation and to develop and control the difference between the high-rate charge cut-off voltage and the associated low voltage cut-in value. While high rate charging is taking place, there is less current passed through the voltage reference element diode than when in the low charging mode. This current depends on the nonlinear current-to-voltage relationship across the voltage reference element and on the conduction characteristics of several transistors which are also nonlinear and temperature-dependent for charging current regulation, thereby limiting precise charging current regulation. Although the Coleman et al. charging circuit produces a desirable controllable hysteresis loop effect, fixed by a diode drop, and enables automatic constant current charging, it is adapted to charge only a specific battery and to adapt the circuit to a different battery voltage requires changing the transistors and their biasing elements.
U.S. Pat. No. 4,426,612 to Wienienski et al. discloses another constant current battery-charging circuit for providing and automatically switching between a full and trickle charge rate in response to a comparison of sensed battery voltage and a reference voltage. This circuit uses an operational amplifier as a level detector to set float and fast switch points and positive feedback through a resistor to provide hysteresis. However, the current through the resistor also flows through other resistors and a potentiometer, all of which affect the reference voltage which is supplied to the level detector from the previous stage. Thus there is a very complex relationship between changes made to the hysteresis resistor and the final amount and levels of hysteresis. For example, changing the reference voltage level via the potentiometer will change the amount of hysteresis, and changing the hysteresis resistor value will change the fast and float switch points via affecting the reference voltage. This complex interaction may place limits on the ultimate amount of hysteresis that can be provided. The Wienienski circuit uses many active elements and requires power supplies not shown in the patent.
Despite the advances in the battery charging art Coleman et al. and Wienienski et al. represent, the need to provide a constant current charging circuit with hysteresis which is easily and readily adaptable to a wide range of battery voltages and capacities remains unaddressed. Further, there is a need for an adaptable, uncomplicated constant current charging circuit which does not depend upon nonlinear element or device characteristics nor upon thermal effects for charging current regulation.